1. Field of the Invention
The present invention relates to a method and a device for inspecting the pattern of a printed circuit board, and more particularly to a technique in which an inspection mode is selected depending on the location of pattern defects on the printed circuit board.
2. Description of the Background Art
Printed circuit boards are employed in the field of electronic engineering for mounting and interconnecting electronic elements. Such boards are provided with conductive wiring patterns on one or both sides of insulating boards and with a large number of through holes piercing the insulating boards. Various types of optical visual inspection devices or pattern inspection devices have been employed in order to inspect whether or not the conductive pattern and the through holes are formed accurately within a tolerance.
FIG. 16 is a partial plan view showing an exemplary conductive pattern which is provided on a printed board. The conductive pattern includes wiring patterns 901, lands 902, and shielding portions 903 for electrostatically shielding mounted electronic parts. Through holes (not shown) are formed in the respective lands 902. An optical pattern inspection device reads images of the conductive pattern and the through holes, and performs pattern inspection of the printed board on the basis of the images. The conductive pattern to be inspected may be a mixed pattern, which is provided with a source or grounding conductive pattern and a signal wiring pattern in a multilayer printed board, as well as an SMT (Surface Mount Technology) lead terminal pattern such as illustrated in FIG. 16.
A conventional pattern inspection device is structured so as to inspect the entire surface of a board according to a single inspection rule. Therefore, pinholes exceeding a prescribed size, for example, are judged as "defects" regardless of their location on the board surface.
However, even if the defects on the board are of the same type, their, degree of importance in quality control varies depending on the location of the defects thereof. For example, even a fine defect is liable to become a critical defect in the wiring area 910 of FIG. 16, while such a fine defect does not significantly influence the quality of the printed board in the central area 911 of the shielding portion 903. Thus, printed boards which are judged as defective by a conventional pattern inspection device include those boards that are usable; i.e., nondefectives. For example those having a fine defect only in the aforementioned central area 911. Therefore, a complicated operation is required in order to confirm whether printed boards being judged as defective are actually non-usable defectives or are printed boards which may be considered nondefective. Such and operation leads to a low output of the entire inspection process.